The invention further relates to a product extruded with the method.
U.S. publication 3,314,108 discloses an extruder comprising two conical stators and a conical rotor rotatably arranged between the stators. The plastic material to be extruded is supplied via two different conduits to different sides of the rotor. The rotor is provided with helical grooves by means of which the material is transferred towards an orifice at the narrower end of the extruder. The flows of material on different sides of the rotor exert a force on the rotor, whereby the bearings of the rotor must be very firm. Further, the clearances between the rotor and the stator should be adjusted carefully so as to prevent a back flow of the extrudable material back toward the material inlet. A small clearance is naturally a difficult structure to implement, and excess friction is easily generated between the rotor and the stators. On the other hand, a bigger clearance would add to the back flow and thereby reduce extruder output.
EP 0,422,042 discloses an extruder with a plurality of conical stators and conical rotors rotatably arranged between them. The extrudable material is fed via one conduit to the forward end of each rotor, the rotor comprising, at the inlet, holes through which the material can also flow to the other side of the rotor. The material is transported to the end of the extruder by means of grooves arranged on both sides of the rotor. The different sides of the rotor are also subject to great forces in this device, requiring firm bearings. A further problem is a back flow over the flights between the grooves back toward the material inlet and an accurate arrangement of clearances between the rotors and stators.
U.S. Pat. No. 3,689,181 discloses an extruder comprising a plate-like rotor and a plate-like stator. Recesses and protrusions are arranged in the rotor and the stator so as to improve the mixing of the material. The material is extruded from the extruder mainly by centripetal force, making the extruder quite ineffective. Furthermore the structure of the extruder is such that it requires very firm bearings.
GB 2,202,783 describes an extruder with a plurality of partially conical rotors on top of each other. The rotors are provided with grooves that transport the extrudable material out of the extruder. The grooves may be provided with mixing means, e.g. rods, to reduce the cross-sectional area of the groove. These means mix the material, but they significantly hamper material flow and thus reduce extruder output.